Digital communication networks today need to be interoperable. Various equipment and networks may use different types of timing protocols to provide timing in the network. When different timing protocols are used in or between networks, it may be necessary to map the timing quality levels between the protocols. Two widely used timing protocols are ITU-T G.781 and IEEE 1588.
The ITU-T G.781 specification specifies a library of basic synchronization distribution building blocks and a set of rules by which they are combined in order to describe a digital transmission equipment's synchronization functionality. The ITU-T G.781 specifies clock quality levels. The IEEE 1588 protocol is a protocol used to synchronize clocks throughout a computer network. On a local area network it achieves high levels of clock accuracy, making it suitable for measurement and control systems. The IEEE 1588 specification defines a clock quality 3-tuple, of which one of the fields is clock class. This clock class has a range of values of [0 . . . 255], and the values may be assigned to various clock traceabilities relative to a primary reference. The IEEE1588 standard leaves many of the clock class values as “reserved” or “profile specific”.
The ITU-T G.8265.1 specification redefines some of the reserved clock class values to map to ITU-T G.781 quality levels. Other specifications may further define or redefine the clock class values to suit specific needs of an industry or product using the IEEE 1588 protocol for time and/or frequency recovery. A IEEE 1588 clock vendor would have to design the clock to work with specific clock class definitions and may not be compatible with clocks using other definitions. Furthermore, when relating IEEE 1588 clock class values to ITU-T G.781 quality levels, different specifications may use different mappings and mappings may conflict from standard to standard. Finally, there may be different meanings for the quality level to clock class mappings in each direction, that is, which clock class is transmitted based on a received quality level and which quality level is transmitted based on a received clock class. These mappings may become complicated and vary between IEEE 1588 profiles and standards. It may become complicated for vendors to design a IEEE 1588 clock to conform to multiple profiles and for users to select the proper profile based on their deployment.